1. Field of the Invention
The present invention relates generally to variable displacement compressors. More particularly, the invention relates to an improved mechanism for controlling the inclination angle of the wobble plate according to the pressure difference a crank case and a suction chamber, so as to properly control the compression displacement.
2. Related Art Statement
In conventional variable displacement compressors, a refrigerant gas is sucked into a suction chamber, and is compressed in a cylinder bore. It is then discharged through a discharge port which is formed in a discharge chamber. The compression displacement is controlled by the inclination angle of a wobble plate. The refrigerant gas is also used to adjust the internal pressure in each one of the associated chambers. As the refrigerant gas is discharged into a crank case from the discharge chamber, the internal pressure in the crank case is caused to raise. In contrast, the refrigerant gas in the crank case is discharged into the suction chamber, in order to reduce the internal pressure in the crank case, for de-pressurizing the crank case. The internal pressure in the suction chamber and the crank case act on a piston, which is supported by the wobble plate, and the inclination angle of the wobble plate is caused to vary accordingly. As the pressure in the crank case is caused to rise, the inclination angle of the wobble plate is caused to decrease accordingly, and the compression displacement of the compressor is also decreased. On the other hand, as the internal pressure in the crank case is caused to drop, the inclination angle of the wobble plate is increased, and the compression displacement of the compressor is also increased.
FIG. 7 illustrates such a conventional compressor. In this compressor, a support member 101 is integrally rotatable and secured to a drive shaft 100. An elongated hole 108 is formed in the supporting member 101. The elongated hole 108 serves as a guide for slidably holding a link pin 109. A drive plate 102 is integrally rotated with respect to the support member 101, and is inclined according to the sliding movement of the link pin 109, along the elongated hole 108. A wobble plate 103 is coupled with the drive plate 102. The wobble plate 103 does not rotate synchronously with the drive plate 102, via a stopper 104. As the wobble plate 103 is caused to swing, due to the rotational movement of the drive plate 102, the wobble plate 103 drives a plurality of pistons 105, to reciprocate in the circumferential direction, in order to compress the refrigerant gas.
The internal pressure Pc in the crank case 106 acts on the rear surface of the piston 105, and the internal pressure Ps in the suction chamber 107 acts on the front surface thereof. The force applied on the piston 105 is transmitted to the wobble plate 103, and is then transmitted to the drive plate 102. The link pin 109 is caused to slide along the elongated hole 108 under the force applied on the drive plate 102. Therefore, the inclination angle of the drive plate 102 will be shifted. The inclination angle of the wobble plate 103 is accordingly varied with respect to the difference between the internal pressure on the both chambers 106 and 107. Therefore, the inclination angle of the wobble plate 103, which determines the piston stroke, is set appropriately by controlling the compressed displacement of the compressor.
As illustrated in FIG. 8, a passage 112 interconnects the crank case 106 and the discharge chamber 110. A throttle 111 is disposed in the passage 112, for preventing pressure leakage from the crank case 106 to the discharge chamber 110. Therefore, the internal pressure in the crank case 106 is not allowed to drop suddenly. Consequently, the inclination angle of the wobble plate 103, which determines the piston stroke, is appropriately maintained by controlling the compressed displacement of the compressor.
In this design, the compressor has the throttle 111 between the crank case 106 and the discharge chamber 110, and both chambers 106 and 110 are interconnected by the passage 112. The refrigerant gas leaking from the discharge chamber 110 into the crank case 106 cannot be completely eliminated. As a result, its cooling ability is decreased and its power loss is increased.
To achieve the foregoing objects, Japanese Unexamined Patent Publication No. 62-191673 discloses a conventional compressor. As illustrated in FIG. 9, the compressor has a switch valve 117 which selects an appropriate passage between a first and second passages 114 and 115, which interconnect the crank case 106 and the suction chamber 107, respectively, as well as between a third passage 116 which interconnects the crank case 106 and the discharge chamber 110, for closing the selected passage. The switch valve 117 is caused to switch based on the force corresponding to the pressure in the discharge chamber 110 and the suction chamber 107.
When the internal pressure Pd in the discharge chamber 110 is higher than the predetermined value (the value is set based on a normal operation of the compressor with normal load), the switch valve 117 is shifted to close the second and third passages 115 and 116. Therefore, even when the compressor is operated under maximum compression displacement, the pressure leakage from the discharge chamber 107 into the crank case 106 is prevented by the third passage 116, which is provided in the crank case 106. Thus, if the internal pressure in the suction chamber 107 were caused to drop below the predetermined value, a switch valve 117, which is disposed in the first passage 114, is activated to close the first passage 114. When the internal pressure in the suction chamber 107 is higher than the predetermined value, the switch valve 117 is activated to open the first passage 114, so as to maintain the appropriate internal pressure Ps in the crank case 106.
When the internal pressure Pd in the discharge chamber 110 is smaller than the predetermined value (the compressor is operated under low load), the switch valve 117 is shifted to close the first passage 114, and to open the second and third passages 115 and 116. A throttle 118 is disposed in the second passage 115. The throttle 118 causes the pressure in the crank case 106 through the suction chamber 107 and the second passage 115, to be lower than that in the discharge chamber 110 through the crank case 106 and the third passage 116. The inclination angle of the wobble plate 103 is shifted by the predetermined value in order to prevent the compression displacement to be deceased.
In the compressor described above, the objective is to keep the internal pressure generally constant in the crank case 106, when the compressor is operated at the maximum displacement. Therefore, when the internal pressure in the discharge chamber 110 is increased beyond the predetermined value, the third passage 116 is caused to close temporarily. As a result, the discharge chamber 110 and the crank case 106 are isolated from each other even if the compressor were not operated at the maximum displacement. The increase of the internal pressure in the crank case 106 is based on the blow-by gas, alone. This causes the response time for decreasing the magnitude of the inclination angle of the wobble plate 103 to be lowered.